Electro-optical system



H. E IVES ELECTRO-OPTICAL SYSTEM Mach 14,; 1939.

Filed March 4, 1936 w w W HEI/VES Patented Mar. 14, 1939 UNITED ST ATENT OFFQE ELECTRO-OPTECAL SYSTEM Herbert E. Ives, Montclair, N. J., assigner to Bell Telephone Laboratories,

Incorporated, New.

York, N. Y., a corporation of New York Application March 4,

4 Claims.

from one scanning period to the next.

These objects are accomplished in the preferred embodiment of this invention, which is hereinafter described in detail, by providing a target or screen upon which an image of an object to be televised is focussed by a suitable lens system and which target is scanned by a beam of light, which beam may be produced by the action of a cathode ray beam impinging upon a fluorescent screen. The target or screen preferably comprises a mosaic consisting of a number of elements, each element including a photoemissive member connected to an elemental high resistance photo-conducting member. The photo-emissive and photo-conducting members are preferably separated by an opaque insulating material, such as colored glass, which is pierced by wires joining the photo-emissive members to the corresponding photo-conducting members. The radiations from the object directed upon the photo-emissive members cause these members to emit electrons to an anode, each photo-emissive member thus acquiring a positive charge, the amount of the charge depending upon the lighttone value of the corresponding elemental area of the object. Because of the high resistance of the corresponding photo-conducting members, these charges do not escape to the external circuit, the terminals of which are connected to the anode and the photo-conducting members. The moving beam of light scans successively the photo-conducting members making them conducting in turn. The charges escaping in turn to the external circuit form a succession of current pulses corresponding to the light-tone values of the elemental areas of the object or field oi View.

Due to the fact that the photo-emissive members are illuminated continuously by radiations from the object, it is possible to store up on each member a large charge, which charge when allowed to escape to the external circuit produces 1936, Serial No. 67,011

a large current pulse. charge from leaking ofi to the external circuit before the proper photo-conducting member is scanned, the photo-conducting members should be made of a material having high specific resistanee in the dark.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof, in which:

Fig. 1 is a diagrammatic representation of a television system including the invention;

Fig. 2 is an enlarged perspective view of a portion of the system shown in Fig. 1; and

Fig. 3 shows a modification of the system of Fig. l.

Referring more particularly to the drawing, the system of Fig. l comprises in general a television transmitting station T, suitable connecting media L, and a television receiving station R.

There is provided at the transmitting station T a cathode ray tube IG having a lgas-tight enclosure I I located at one end thereof, in which is mounted a composite target or screen S and a mesh electrode I2. The target S is mounted on the wall I9 of the enclosure II carrying the fluorescent screen I3. Associated with the cathode ray tube is a suitable means for forming an image of an object or eld of view O on the target S and means, such as the cathode ray beam of the cathode ray tube II) impinging upon the Fluorescent screen I3, for causing the target to be scanned with a beam of light.

The cathode ray tube I0 is provided with means, such as a cathode I4 and an anode I5, for producing a thin beam of electrons which is directed towards the fluorescent screen i3. The cathode beam of the tube I0 is deflected in two directions at right angles to each other and at such relative speeds in the two directions that the screen I3 is impinged upon by a moving pen-- cil of electrons which moves across the screen in a series of parallel lines in a time interval within the period of persistence of vision. Deection of the beam in the two directions is obtained in any desirable manner, such as by supplying the two pairs of deflecting plates I6 and I'l with currents of saw-toothed wave forms, the frequencies of these currents being such that the fluorescent screen I3 is completely scanned in a time interval within the period of persistence of vision. This moving pencil of electrons thus generates a moving beam of light which scans one surface of the composite target or screen S.

For a clearer understanding of the composi- In order to prevent the tion of the target or screen S, reference is made to Fig. 2 which shows an enlarged perspective View of the assembly within the enclosure I I with the glass portions broken away. The target S comprises a mesh grid I8 of any suitable conducting material, such as nickel, which is preferably mounted on the end wall I9 of the cathode ray tube l@ adjacent the fluorescent screen I3, a large number of photo-conducting members 22 filling the interstices of the mesh grid I8, an opaque plate 2Q of any suitable material such as dark glass and a large number of photo-emissive globules 2l, corresponding in number to the photo-conducting elements 22, each elemental globule being joined through the glass plate to its corresponding photo-conducting member 22 by an appropriate connection 23. The plate 20 is made opaque so that the light from the object will not strike the photo-conducting members 22. ssociated with the target or screen S and adjacent thereto is anV anode member I2 which may constitute a mesh grid of nickel or any other suitable material.

The anode member i 2 is connected to the positive terminal oi the battenr 24 and the grid member l2 is connected to the negative terminal of the battery 24 through the resistance 25 which is connected in the input circuit of a suitable amplifier 2S.

Associated with the enclosure Il is an optical means for directing an image oi an object or field of view O upon the target or screen S. 'Ihe object or eld of view O is illuminated by radiations from a suitable source (not shown) and radiations reected from the object are gathered by an optical system, represented generally by the lens 2S, and projected through the interstices of the anode I2 onto the-photo-emissive surface of the target S.

The operation of the cathode ray 'tube I and its associated scanning device located at one end thereof may be better understood by considering the operation of a single element of the mosaic screen S. Light from the object falling on a photo-emissive globule 2! releases electrons which are attracted to the anode member I2. The elemental photo-emissive globule 2| is thus charged up to a positive potential, the value of the charge depending upon` the intensity of the illiu'nination. This .charge does not escape because of the high resistance of the elemental photo-conducting member 22. At intervals of approximately one-twentieth of Aa second,V the cathode ray beam of the cathode ray tube I0 is swept over an elemental area of the fluorescent screen I3, which, giving out light, causes the elemental photo-conducting member 22 to be-V come conducting whereby the charge on theV elemental photo-emissive member 2| is allowed to escape toy the external circuit including the resistance 25, thus giving a current pulse to the input circuit of the amplifier 25. If the complete screen is considered, there is produced a succession of current pulses as the elemental photoconducting members are Vscanned vin turn, thus producing a television or image current.

The image current whch flows. through the resistance 25, Vafter being raised to the desired power level by the device 2S, which may comprise a multi-stage amplier, is transmitted over suitable connecting means L to a remote station including a receiver R. In line carrier or radio transmission the amplied image current is used to modulate a carrier current of the proper frequency for transmission.

The receiving station R may comprise an amplier 21, the output circuit of which includes the television receiver 28. Any suitable receiver for the purpose may be used. A satisfactory receiver utilizing a glow discharge lampA and a scanning disc is disclosed in U. S. Patent 1,728,122, September l0, 1929, to Horton. A suitable cathode ray tube receiver is disclosed in application Serial No. 466,067 of J. B. Johnson, filed July '7, 1930.

The image current flowing through the system oi Fig. 1 will be quite large because the photoemissive members 2l are illuminated continuously by radiations from the object. It is thus possible to store up a large charge which does not leak off to the external circuit until the scanning beam passes over the discharging photo-conducting member. In order to reduce the leakage through the photo-conducting members to a negligible amount, it is necessary to make them of a suitable photo-conducting material having a higher specic resistance in the dark than the ordinary selenium. 'Ihe material should also have a quicker response than selenium. A material possessing both of these desirable properties is mercurio iodide (I-IgIz) but my invention is not limited to the use of this material as any photoconducting member possessing these properties can be used in its place. Any suitable photoemissive material, such as potassium hydride, may be used for the globules 2l.

In the modification shown in Fig. 3, the mosaic target S andthe mesh anode I2 are mounted in an enclosure 30 whichis adapted to be a separate unit from the cathode ray tube 3l. The moving pencil of light generated by the iluorescent screen of the tube 3l is caused to scan the surface of the photo-conducting element by means of the optical system represented generally by Vlens The description and operation of the system shown in Fig. 3 is otherwise similar to that described above with reference to Fig. l. The taiget or screen S may be mounted on one wall of the enclosure 30 or it might be supported in. the space between the walls. The anode I2 may take the form oiV a transparent metal layer mounted on one wall of the enclosure 30.

Other changes may be made without departing from the spirit ofV principles of the invention as hereinbefore expressed, the scope of which is defined bythe appended claims.

What is claimed is:

l. An electro-optical system including a cathode ray tube comprising a cathode, an anode, a fluorescent screen, an assembly adjacent to and cooperating with said cathode ray tube compising a composite target and a light transmitting anode, said target comprising a layer of discrete photo-conducting members of high specic resistance, a film of discrete photo-eniiseive members, and means for conductively connecting each photo-conducting member to its corresponding photo-emissive member, means for imaging an object eld upon said photo-emissive members to cause emission of electrons from each photo-emissive member to the light transmitting anode to thereby impart a positive charge to each member in accordance with the lighttone value of the corresponding elemental area of an object or field of View, said photo-conducting members being of such high specic resistance in the dark that they act substantially as insulators when the light thereupon is weak to prevent passage of the charges through the photo-conducting members, means for preventing the photo-conducting members from being exposed to radiations from the object, and said fluorescent screen being mounted closely adjacent and parallel to said layer of photo-conducting members to cause the light from said screen to render them conducting in turn each for a period very short compared to the period when said positive charge is accumulating so that said positive charges are successively caused to pass through the photo-conducting members to thereby control the flow of an image current.

2. An electro-optical system comprising an array of photo-emissive elements, means for simultaneously imparting charges to all of the various photo-emissive elements in accordance with the light-to-ne values of the corresponding elemental areas of an object or eld of view, a photo-conducting element electrically connected to each photo-emissive element, a conducting member connected to all of the photo-conducting elements in common, said photo-conducting elements being of such high specific resistancey in the dark that they act substantially as insulators when the light thereupon is Weak to prevent leakage of the charges through the photoconducting elements to the conducting member, and means for scanning said photo-conducting elements with a beam of light of suicient intensity to allow the charges to pass through the photo-conducting elements to the conducting member to thereby set up an image current.

3. An electro-optical system comprising an array of photo-emissive elements, means for simultaneously imparting charges to all of the various photo-emissive elements in accordance With the light-tone values of the corresponding elemental areas of an object or field of View, a photo-conducting element electrically connected to each photo-emissive element, a conducting member connected to all of the photo-conducting elements in common, said photo-conducting elements; being of. such high speciiic resistance in the dark that they act substantially as insulators when the light thereupon is weak to prevent leakage of the charges through the photo-conducting elements to the conducting member, and means for scanning said photoconducting elements with a beam of light of suicient intensity to allow the charges to pass through the photo-conducting elements to the conducting member to thereby set up an, image current, said conducting member comprising a mesh struc-ture in the interstices of which the photo-conducting members are mounted.

4. An electro-optical system comprising an array of photo-emissive elements, means including an anode member towards which the electrons emitted from said photo-emissive layer are attracted for simultaneously imparting charges to all of the Various photo-emissive elements in accordance with the light-tone values of the corresponding elemental areas of an object or field of view, a photo-conducting element electrically connected to each photo-emissive element, a conducting member connected to all of the photoconducting elements in common, said photoconducting elements being of such high specic resistance in the dark that they act substantially as insulators when the light thereupon is weak to prevent leakage of the charges through the photo-conducting elements to the conducting member, and means for scanning said photo-conducting elements with a beam of light of suiicient intensity to allow the charges toy pass through the photo-conducting elements to the conducting member to thereby set up an image current.

HERBERT E. IVES. 

